Connected: An Internet Encyclopedia
RFC 1123 - 5.2.2 Canonicalization: RFC-821 Section 3.1

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### 5.2.2 Canonicalization: RFC-821 Section 3.1

The domain names that a Sender-SMTP sends in MAIL and RCPT commands MUST have been "canonicalized," i.e., they must be fully-qualified principal names or domain literals, not nicknames or domain abbreviations. A canonicalized name either identifies a host directly or is an MX name; it cannot be a CNAME.

Next: 5.2.3 VRFY and EXPN Commands: RFC-821 Section 3.3

Connected: An Internet Encyclopedia
RFC 1123 - 5.2.2 Canonicalization: RFC-821 Section 3.1